The invention is based on a fuel injection pump for internal combustion engines as defined hereinafter. In such a fuel injection pump, the quantity of fuel supplied to the injection nozzles per pump piston stroke and there attaining injection into the cylinders of the engine is metered precisely by means of an electrical final control element. It has been found that the accuracy with which the fuel quantity can be metered depends not only on the constancy in material properties of the fuel supplied, but also on the functional capability of the final control element.
In a known fuel injection pump of this generic type (German Offenlegungsschrift 37 39 198; U.S. Pat. No. 4,869,218) the temperature of the fuel delivered to the injection nozzles is ascertained by moving a flow of fuel, which is permanently diverted from the pump interior, and the temperature of which is at a known ratio to the temperature of the fuel supplied to the injection nozzles by the pump piston, past a temperature sensor; the output signal of the temperature sensor is used as a corrective variable in the control variable supplied to the electric final control element for actuating the quantity control device.
However, this does not take into account the fact that the values of the control variables supplied to the control unit entail systematic errors, if the electrical parts of the final control element that embody the measurement, such as the potentiometer resistor tracks that are to be scanned by wipers and are exposed to the fuel, are altered by the fuel, and in particular if fuel additives, which are increasingly used and the composition and effects of which are increasingly complex, cause these parts to undergo an incalculable drift over time, bringing about an incorrect setting in fuel metering.
In a fuel injection pump known from German Offenlegungsschrift 37 04 578; U.S. Pat. No. 4,873,956, the attempt is made to keep additives, in the Diesel fuel that reaches the final control element chamber, away from it by providing that the control element chamber communicates with the pump interior only via a narrow bearing gap on the final control element shaft.
In particular, this is intended to keep the water, which causes corrosion in the parts of the final control element carrying an electric potential, away from the final control element chamber.
A close bearing fit, which is necessary to attain this object, and deposits resulting from fuel additives, can on the one hand impair the function of the final control element as a result of friction; on the other, additives can once again settle on the potentiometer, which is likewise disposed in the final control element chamber, so that not only do they adulterate the electrical values but they also cause corrosion.